Add additional capture transform tests.

[rxwei]

• Add a test where the capture transform produecs a Substring from a Substring.
• Add a test where the capture transform wraps a Substring in an Optional.

[rxwei]

From #469:

Similarly for when that capture-transform appears inside an Optionally or is adding or removing a layer of Substring optionality.

In this patch, it's impossible to write a test that removes a layer of Substring optionality because we don't have #470 yet. That is, a capture transform can only take a Substring today. I'll test this in that PR.

[rxwei]

@swift-ci please test